1. Field of the Invention
The present invention relates to the electroplating of chromium and chromium alloy coatings on titanium substrates and, more particularly, to surface preparation and post-electroplating techniques for promoting adherence of such electrodeposits to the titanium substrate surface.
2. Description of the Prior Art
Titanium alloys have been extensively used in gas turbine engines for over two decades as a result of their favorable strength-to-weight ratios. As gas turbine engine technology has advanced over the years, the demands placed on these alloys have become increasingly severe and it has been necessary to provide titanium with suitable coatings to alleviate such potential problems as erosion, corrosion, fretting and galling. A coating system found to fulfill these and other requirements for a successful coated titanium article is a chromium coating containing a relatively small but effective amount of molybdenum. In general, molybdenum in amounts up to about 5 weight percent has proved effective in increasing wear resistance and corrosion resistance of the coating. Usually, the molybdenum content selected depends upon the particular properties desired in view of the intended service application of the coated article. Generally, increasing coating ductility will be evident with increasing molybdenum content with the actual maximum molybdenum content being determined by the wear and oxidation corrosion resistance required. Excessive quantities of molybdenum can be expected to result in poor oxidation characteristics.
Although the chromium molybdenum coating can be deposited on titanium substrates by a number of methods, including pack and slurry techniques, vapor deposition and sputtering, a particularly convenient method involves electrodeposition from an aqueous bath containing conventional chromium plating salts (for example, CrO.sub.3) and, in addition, a source of dissolved molybdenum such as molybdic acid, sodium molybdate and ammonium molybdate. Small amounts of a catalyst, typically sulfate ions, are also present in the bath. A problem associated with the electrodeposition of chromium alloys on titanium substrates has been one of suitable adhesion of the asdeposited coating to the substrate surface. Another problem is that interdiffusion of the chromium alloy deposit with the substrate during any heat treatment can affect both the wear resistance and mechanical properties of the article.
Prior art workers have devised various approaches to obtaining advantageous results as reflected in various U.S. patents. Glover et al., U.S. Pat. No. 3,439,188, discloses solvent cleaning and grit blasting, when copper plating with the aid of ultrasonics, followed by heat treatment in the 1650.degree.-1725.degree. F. (900.degree.-930.degree. C.) range. Missell et al., U.S. Pat. No. 2,825,682, discloses pretreatment by forming a chromium conversion coating. Ogden, U.S. Pat. No. 3,560,274, discloses a pretreatment of chemical polishing; deposit of a relatively thin layer; and, posttreatment by diffusion annealing at temperatures of 1600.degree.-1900.degree. F. (870.degree.-1140.degree. C.), whereby complete diffusion was achieved. Raymond, U.S. Pat. No. 3,691,029, discloses a two-step process wherein a first thin plating is diffusion bonded at 1600.degree.-1900.degree. F. (870.degree.-1140.degree. C.), and followed by further electroplating.
In electrodepositing chromium molybdenum and other alloy coatings on titanium substrates, it has been recognized that, ideally, the substrate surface should be clean, free of interface contamination and activated so that the electrodeposit will adhere without further treatment. Unfortunately, titanium is not easily activated and maintained for any extended period of time in that condition as a result of its rapidly forming oxides. The most sophisticated surface conditioning treatments, such as conversion of the surface to hydrides or complexing the surface with tartrates to minimize oxide formation, have not resulted in satisfactory bond strengths. Other treatments such as abrading while plating or inert atmosphere plating have not significantly improved the results. For high performance gas turbine engine applications, it has been demonstrated that suitable adhesion in the service environment cannot be obtained without a diffusion heat treatment after plating. Of course, it is well known that heat treating titanium alloy substrates, particularly alloys such as Ti-6Al-2Sn-4Zr-2Mo, can alter their mechanical properties by metallurgical changes and alleviation of desirable residual stresses. Thus, it is preferred to have a hardfacing method which avoids higher temperatures.
Further, it is undesirable to use a method which uses sandblasting since grits thereby become embedded in the substrate surface. However, light blasting often may not by itself give adequate preparation. And chemical polishing is not particularly desirable when fine dimensions are being maintained.